I think getting parent document name in search result transformation might be quite inefficient: i.e. if you have search result page contains let say with 10 documents - you might end up with 10 queries to get a parent name. What you can do instead is to form your Document Page Title correctly for example:
This way it will be also good for SEO purposes so you dont have duplicated page titles (very important for google).
Then you can basically say for your smart search that the title field is documentpagetitle:
and get it in trasformation like <%# GetSearchValue("title") %>
It is a good practice to check your index with Lucene Index Toolbox. All
you smart search indexes located in CMS\App_Data\CMSModules\SmartSearch. You can see what are the fiedl names right away, instead of guessing.
P.S. I was under impression the documentpagetitle should be in index by default, by I dont see it in kentico 9, may in 10 it is already there.